a) Field of the Invention
The present invention relates to a multi-shell emulsion particle which is useful as an additive to coating compositions for paints, paper coating and information recording papers, and has a penetrating pore in the interior of the particle.
b) Description of the Related Art
In recent years, high molecular weight polymer particles prepared by emulsion polymerization have been studied for utilization as organic pigments in various fields.
The most commonly used polymer particles are homogeneous and solid, emulsion-polymerized polystyrene particles having a diameter of 0.2 to 0.5.mu.. For example, Japanese Laid-Open Patent 59741 (1984) discloses a process for copolymerizing an unsaturated carboxylic acid and a vinyl monomer in the presence of an anionic surface active agent and/or a nonionic surface active agent to prepare a copolymer emulsion wherein more than 90% of the particles have a diameter of 0.20 to 0.28.mu..
The patent also describes that the copolymer emulsion can be used as an organic pigment in applications such as paper coating and paints. However, the organic pigment obtained by the process fails to provide sufficient hiding power, brightness and gloss, and no practical advantage is obtained unless it is used in a large amount. Consequently, the pigment leads to problems such as high cost and weight increase.
In order to further improve hiding power, brightness and gloss, conversion from the above homogeneous and solid particles to pored emulsion particles has recently been proposed in U.S. Pat. No. 4,427,836. The pored emulsion particles are obtained by mixing a core material, a polymer dispersion, formed by copolymerizing at least 5% by weight of an unsaturated carboxylic acid, with at least one of a monoethylenically unsaturated shell monomer for forming a shell polymer, carrying out emulsion polymerization, neutralizing the resulting emulsion with an aqueous, volatile base to swell the core polymer and thereafter drying to form minute openings in the interior of the particles. When the emulsion particles thus obtained are used for an organic pigment, the hiding power and brightness have been found to be better, though not satisfactory, than those of the above homogeneous and solid particles.
The swelled core polymer in the interior of the particles contain moisture. The moisture evaporates by drying the particles to make the void. However, the moisture must transmit through the layer of shell polymer of particles and hence evaporation becomes difficult. Larger voids lead to larger amount of moisture contained and result in a disadvantage of decreased drying ability. When the above particles are used as an organic pigment for paper coating, remarkable improvement cannot be observed on printability such as receptivity to inks, ink-setting ability and blistering property.
On the other hand, Japanese Laid-Open patent 281886 (1988) discloses an example for improving coloring sensitivity of a heat-sensitive recording paper by employing the above particles having minute openings for the undercoat of the paper to enhance heat insulation property. The heat-sensitive recording paper prepared by the process has some improvement in color developing sensitivity as compared with recording papers prepared by using inorganic pigments or urea resin. However, no improvement is observed on the foreign matter adhesion to a thermal head and sticking.
As mentioned above, emulsion particles having minute openings in the interior have been improved in view of hiding power and brightness as an organic pigment. On the other hand, lack of oil absorbing property and gas permeability due to the particle structure is a serious disadvantage as a pigment and leads to problems in practical use.